Positioning accuracy of CNC machine tools refers to the positional accuracy that can be achieved by the movement of each coordinate axis of the machine tool under the control of the CNC device. The positioning accuracy of CNC machine tools can also be understood as the movement accuracy of the machine tool. Ordinary machine tools are manually powered, and the positioning accuracy is mainly determined by the reading error, while the movement of CNC machine tools is achieved by digital program instructions, so the positioning accuracy is determined by the CNC system and mechanical transmission errors.
CNC machine tool is the abbreviation of numerically controlled machine tool. It is an automated machine tool equipped with a program control system. The control system can logically process programs with control codes or other symbolic instructions, decode them and express them with coded numbers, which are input into the CNC device by Nanjing Fourth Machine Tool Co., Ltd. via an information medium. After calculation and processing, the CNC device sends various control signals to control the movement of the machine tool and automatically process the parts according to the shape and size required by the drawing.
The movement of each moving part of the machine tool is carried out under the control of the CNC device. The precision that each moving part can achieve under the control of the program instructions directly reflects the precision that can be achieved by processing the parts. Accuracy is a very important test content.
1. Linear motion positioning precision detection
The positioning accuracy of linear motion is generally achieved under no-load conditions of the machine tool and worktable. According to national standards and the provisions of the International Organization for Standardization (ISO standards), the detection of CNC machine tools should be based on laser measurement. In the absence of a laser interferometer, ordinary users can also use a standard scale with an optical reading microscope for comparative measurements. However, the accuracy of the measuring instrument should be 1-2 levels higher than the accuracy of the measured object.
In order to reflect all errors in multiple positioning, the ISO standard stipulates that each positioning point is calculated based on five measurement data, and the average value and dispersion are calculated. The dispersion band constitutes the dispersion band of the positioning point.
2. Linear motion repeat positioning accuracy detection
The instruments used for testing are the same as those used to test positioning accuracy. The general detection method is to measure three positions near the midpoint and two ends of each coordinate stroke, position each position with a rapid movement, repeat the positioning 7 times under the same conditions, measure the value of the stop position and find the maximum difference. in readings. Half of the largest difference between the three positions, accompanied by a positive and negative sign, is used as the repeated positioning accuracy of the coordinates. This is the most basic indicator that reflects the accuracy and stability of axis movement.
3. Precision detection of linear motion return to origin
The accuracy of returning to the origin is basically the accuracy of repeated positioning of a special point on the coordinate axis, so its detection method is exactly the same as the accuracy of repeated positioning.
4. Linear motion reverse error detection
The inverse error of linear motion, also called the amount of lost momentum, includes the inverse dead zone of the coordinate axis power transmission chain drive part (such as servo motor, motor servo-hydraulic and stepper motor, etc.), each mechanical pair of motion transmission A complete reflection of errors such as backlash and elastic deformation. The larger the error, the lower the positioning accuracy and repeated positioning accuracy.
The method of detecting the reverse error is to move a certain distance in the forward or reverse direction in advance in the stroke of the measured coordinate axis and use this stopping position as a reference, and then to give a certain motion command value in the same direction to move it a certain distance. Then move the same distance in the opposite direction and measure the difference between the stopping position and the reference position. Take several measurements (usually 7 times) at three positions near the middle and both ends of the stroke, find the average value at each position, and use the maximum value among the average values as the inverse error value.
5. Positioning precision detection of rotary worktable
Measuring tools include standard turntables, angular polyhedra, circular arrays and parallel light tubes (collimators), etc., which can be selected according to specific circumstances. The measuring method involves rotating the workbench forward (or backward) at an angle and stopping, locking and positioning it. Use this position as a reference, then quickly rotate the workbench in the same direction, lock it and position it every 30 seconds. and measure. The forward rotation and reverse rotation are measured for one revolution each, and the maximum value of the difference between the actual rotation angle at each positioning position and the theoretical value (command value) is the indexing error. If it is a CNC rotary workbench, every 30 should be used as the target position, and each target position should be quickly positioned 7 times in the forward and reverse directions. The difference between the actual position and the target position is the position deviation. then press GB10931-89《CNC machine tool position Calculate the average position deviation and standard deviation according to the method specified in “Position accuracy evaluation method”. The difference between the maximum value of all average position deviations and standard deviations and the sum of the minimum values of all average position deviations. and standard deviations are CNC rotary work. The positioning accuracy error of the platform.
Considering the actual use requirements of dry-type transformers, several equal right angle points such as 0, 90, 180 and 270 are generally measured emphatically. The accuracy of these points must be one level higher than that of other angular positions.
6. Repeated detection of indexing precision of rotary worktable
The measurement method is to select any three positions of the rotating workbench and repeat the positioning three times in a week, then carry out the detection under forward and reverse rotation respectively. The maximum indexing accuracy of the difference between all reading values and the theoretical value at the corresponding position. If it is a CNC rotary workbench, a measuring point should be taken every 30 as the target position, and each target position should be quickly positioned 5 times in the forward and reverse directions, and the difference between the position actual achieved and the target position must be measured. That is, the position deviation, then calculate the standard deviation according to the method specified in GB10931-89. Six times the maximum value of the standard deviation of each measuring point corresponds to the repeated indexing precision of the CNC rotary machine. painting.
7. Precision detection of return to origin of rotary worktable
The measurement method is to return to the origin from 7 arbitrary positions, measure the stopping position, and use the maximum difference read as the precision of return to the origin.
It should be emphasized that the existing detection of positioning accuracy is measured under rapid positioning conditions. For some CNC machine tools with poor power supply system, different positioning accuracy values will be obtained when positioning at different feed speeds. In addition, the positioning accuracy measurement results are related to the ambient temperature and the working state of the coordinate axis. Currently, most CNC machine tools use a semi-closed loop system. Most position sensing components are installed on the drive motor, which drives. within 0.01~0.02mm error in 1m stroke, it is not surprising. This is an error caused by thermal elongation. Some machine tools use pre-stretch (pre-tighten) to reduce impact.
The repetitive positioning accuracy of each coordinate axis is the most basic accuracy index that reflects the stability of the axis movement accuracy. It cannot be assumed that machine tools with low precision can be stably used for production. At present, due to the increasing number of functions of CNC systems, system errors such as step accumulation error, backlash error, etc. are common. can be systematically compensated for the precision of the movement of each injector. Only random errors cannot be compensated and repeat positioning accuracy. This reflects the complete random error of the feed drive mechanism, which cannot be corrected by CNC system compensation. When found to be out of tolerance, the power transmission chain can only be refined and corrected. Therefore, if you are allowed to choose a machine tool, you should choose a machine tool with high repeatability positioning accuracy.
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